The invention relates to a television receiver comprising means for superimposing text on a received television picture, the text being contained within a predetermined area of the display, and means for reducing the contrast of the received picture within the predetermined area.
There are a number of reasons for wishing to superimpose text on a television picture display. It is known to provide, for example on screen message displays to help a viewer select a particular channel or to help in the adjustment of picture contrast, brightness etc. Another form of superimposed text is for the production of sub-titles for the deaf. The subtitles may be transmitted as teletext signals or as data on line 21 of the television signal as in the United States of America. Teletext decoders may also allow the superimposition of a full page of text on a television picture as used in the MIX mode form. MIX mode teletext displays may, depending on the picture content, be difficult to read as the characters may merge into the picture. The same problem could occur with sub-titles and to prevent this it has been specified by the U.S. Federal Communications Committee that the sub-titles appear on a solid black background. This solution is, of course, impossible to implement with MIX mode teletext and, depending on the length of the message, may be impractical with on-screen display messages. In addition even with subtitles the provision of a black box around the subtitle will obscure part of the picture and this may impair the enjoyment of the viewer.
A proposal to overcome the problem of the difficulty of reading text superimposed on a picture is to provide a reduced picture contrast box to contain the text rather than a solid black box. This enables some retention of the picture information presented to the view but causes the text to stand out more distinctly against the background. Japanese Patent Application No. 61-99489 discloses a television receiver in which the contrast of the received picture signal is reduced in an area of the display where the text is to be displayed before the text is superimposed thereon. The area defined extends over a plurality of scanning lines. When producing on screen display messages or sub-titles it may be that only a single character is displayed at one time. Under such circumstances it is not desirable to reduce the picture contrast across the whole width of the display screen. The use of an arrangement such as that shown in the above referenced Japanese Patent Application requires the gain of the luminance control circuit to be switched very quickly and this may be difficult to achieve since it usually involves the charging or discharging of a capacitor. Such an arrangement is not feasible for switching the gain of analogue circuits at a sufficient rate to change the contrast within a television line.